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  In this tutorial, we'll learn about the system design for PasteBin. If you don't know what Pastebin is here goes its definition. Pastebin is a type of online content hosting service where users can store texts to share with others over the internet.  Pastebin system design requirements  Functional Basic - User should be able to paste the text, generate the link and share the link with other users. There should be the max size of the text that our system will support (say 10Mb) Users should be able to provide the custom URL path for the link created. Like if a user wants to create the links  www.pb.com/letlearn, he can give this URL and pastebin should create a link using this URL to share with others. The link or paste expiry feature should be there for each stored text. There can be default expiry time say 1 year but this should be configurable. Personalization means the user should be able to login to track all the pastes created by him and manage them. Non-Functional Durab

  In this tutorial, we'll learn about how tls (or SSL) works in case HTTPS. From the image above it is clear that HTTPS = HTTP + S SL. These days TLS (Transport Layer Security) term is used instead of SSL. A bit of Cryptography Lets learn brief details of different cryptographic algorithms Asymmetric algorithms: In these algorithms, two keys (private and public) are used. Private keys are never shared and kept by the servers and the public keys are shared with the clients. Symmetric algorithms: In these algorithms, same key used by servers and clients for encryption and decryption of data. NOTE: In Assymetric algorithms, encryption and decryption process is costly as compared to Symmetric algorithms because of complex calculations. But those complex calculations are worth it as it provides a high security to prevent any kind of attacks. So in case of HTTPS initial handshaking uses assymetric algorithm, then symmetric algorithm is used for actual data transfer between the server an

 In this tutorial, we'll learn about how to see or unblock blocked contacts in WhatsApp. Steps 1. Open WhatsApp and click on options on top right corner. 2. Select Settings. 3. On the settings page, select Account. 4. On the Account page, click Privacy. 5. On the Privacy page, there are Blocked Contacts option. 6. Click on the Blocked contacts option to see all the blocked contacts. You can click on any contact to unblock it. HOPE YOU LIKE THIS TUTORIAL. FEEL FREE TO COMMENT BELOW IF YOU HAVE ANY DOUBTS. AND STAY TUNED FOR MORE TUTORIAL :) Till then, Happy Learning!!

 In this tutorial, we'll learn about the Observer design pattern as in what it is, in which scenario it is being used and how it works. If you've are not familiar with what are software design patterns and what are its different types check out  this  tutorial. Introduction An observer design pattern is a behavioral design pattern, As the name suggests, the Observer contains observe. So this design pattern is used when one or more entities observe the changes in an entity (or subject).  Usecases Let's learn this with an example, you must be familiar with subscriptions on youTube. When you subscribe to a channel you get notified if some new video is uploaded on that channel. Your subscribing means you are observing the channel for more content. Like you there can be many people who are subscribing to the same channel. So there are multiple observers which are observing a channel (or subject). Basically, the observer pattern moves the pull model to push model which means ins

 In this tutorial we'll learn about one of the most important web application vulnerability known as XSS (i.e Cross Site Scripting) . As the name says this vulnerability occurs when it is possible to run js segment be an user on site different from site of current user . We'll study two types of XSS Reflected XSS Stored XSS Reflected XSS Reflected cross-site scripting vulnerabilities arise when data is copied from a request and echoed into the application's immediate response in an unsafe way. An attacker can use the vulnerability to construct a request that, if issued by another application user, will cause JavaScript code supplied by the attacker to execute within the user's browser in the context of that user's session with the application. The attacker-supplied code can perform a wide variety of actions, such as stealing the victim's session token or login credentials, performing arbitrary actions on the victim's behalf, and logging their keystrokes. Use

 In this tutorial, we'll learn about the singleton design patterns as in what it is, in which use-cases it is being used and how it works. If you've are not familiar with what are software design patterns and what are its different types check out  this  tutorial. Introduction A factory design pattern is a creational design pattern that is related to object creation. This design pattern restricts to create only one object of the class. Whenever the client tries to instantiate a class, if this is the first time we are instantiating it will create an actual object, from next time onwards it will return the same object instead of creating a new one. Usecases  There are a lot of use-cases where this pattern is useful. One of the common use cases is creating a DB connection. If you have a monolithic application and you wanna perform DB operation from multiple places in the app. To do any DB operation you need to connect to the DB first. And we should not be creating a new connection